Control materials (poison), such as control rods, are employed in, for example, nuclear reactors to perform duel functions of power distribution and reactivity control. Power distribution in the core is controlled during the operation of the reactor by manipulation of selected patterns of rods that enter from the bottom of the reactor core. Each control rod in its power distribution function may experience a similar or a very different neutron exposure than other control rods in the control system. Control rods are generally cruciform in cross section and typically comprise a plurality of absorber tubes extending axially in each wing of the rod. In one design, the tubes are filled with boron carbide powder and seal-welded at their ends with end plugs. The powder is separated into sections or segments. In another design, the tubes are filled with capsules of discrete lengths containing the boron carbide powder. A plurality of stainless steel capsules are stacked in each tube with the tubes lying side by side in each wing of the control rod, generally in parallel with the long axis of the rod. These capsules or segments, for example, may have lengths of one foot or more.
Control materials such as control rods having higher worth are important to obtain adequate control for thermal reactors that incorporate mixed oxide fuels and may have an economic benefit for uranium-fueled thermal reactors.
Thermal reactor poison loadings are usually characterized by periodically distributed masses of strong thermal absorbers, particularly boron carbide. Because of the short mean free path of thermal neutrons in such a mass, most of the absorptions occur near the surface of the mass, and the remainder of the poison mass is less effective as a thermal absorber.
This effect can be better understood by a lumped mass of boron carbide in a neutron flux field, where the neutrons are assumed to have a spectral distribution consistent with a thermal reactor spectrum. The thermal neutrons in this field are strongly absorbed by the poison at the surface of the boron carbide (the "onion skin" effect). The resulting neutron distribution in the interior of the poison mass has a much lower fraction of thermal neutron than the external field. Therefore, the absorber in the interior of the poison mass has a much lower neutron absorption rate.
This effect is compounded by the fact that boron carbide is a "1/v" absorber--i.e., its cross section is inversely proportional to the neutron energy; therefore, boron carbide in the center of a poison mass is much less effective at absorbing neutrons than the same material at the surface of the mass due to spectral hardening.
The strength of the control rods in a nuclear reactor helps define the amount of fissile material that may be loaded in the core while assuring that the fission reactions in the core may be curtailed at any time. Stronger control rods permit the loading of a larger fissile inventory without corresponding increases in integral burnable absorbers (e.g., Gadolinia). Similarly, they also ease the introduction of mixed-oxide fuel. Because of the larger thermal absorption cross section of plutonium, such fuel makes conventional boron carbide loaded control rods less effective, not only because of the spectral change due to absorption hardening, but also because of a decrease in the flux in the vicinity of the lumped poison (for a fixed number of neutrons, there is a greater fraction absorbed in the more strongly absorbing discrete fuel masses).
As with all high worth absorbers, higher worth implies faster destruction of absorber atoms. Unless a chain absorber (an element where nuclides transmute with neutron absorption, either directly or indirectly through radioactive decay, to other nuclides with large absorption cross sections) is used, the time span that such a lumped poison may be used is shorter than a lumped poison with a lower worth.